This invention relates generally to ink jet printers and particularly to vector magnetic ink jet printers, otherwise known as "VMIJ" printers.
In a VMIJ printer a stream of ferrofluid ink droplets emitted under pressure from a vibratory nozzle is passed through the respective magnetic fields of successively arranged X and Y electromagnetic deflectors which have the combined effect of causing ink droplets to be deposited upon a recording surface along selected vectors or line segments to form printed characters or graphic plots thereon. Printers of this type are able to form printed characters in cursive fashion at higher speed with greater accuracy and with less waste of ink than other types of ink jet printers such as those which operate on the raster scan principle, whether they use electromagnetic or electrostatic deflection.
In a conventional VMIJ printer the X deflector (which deflects the magnetic ink droplets along a horizontal X axis) is positioned ahead of the Y deflector (which deflects the droplets along a vertical Y axis. In carrying out the present invention, it has been found preferable to reverse this sequence and have the Y deflector precede the X deflector. This is not a necessary condition, however. For convenience, in the present portion of this description, these two deflectors will be referred to simply as "first" and "second" deflectors without specifying their respective deflection axes, it being understood that such axes are in orthogonal relationship to each other and in substantially parallel relationship with the recording surface.
In a VMIJ printer it is desirable that the ink droplets be produced at a rate such that they will follow one another in sufficiently close succession to provide an aerodynamically stable jet stream. When the ink droplets are emitted at this rate, however, not all of the droplets which issue from the nozzle can be utilized for printing characters, and to separate those droplets which are to be used from those which are not needed in printing, the stream of droplets is first passed through a special type of magnetic deflector called "selector" before it reaches the first of the two axial deflectors described above. The present arrangement is such that when the droplets issue from the nozzle in which they are formed, they are aimed at an excess ink collection device called a "gutter" or "catcher." If a droplet is to be used in printing, it is "selected" by being magnetized by the selector. An "unselected" droplet (one not to be used in printing) is not magnetized as it passes through the selector. The selector exerts some deflecting action upon those droplets which it magnetizes, sufficient to divert such droplets from a course aimed at the catcher to one which will enable these droplets to reach the recording surface after passing successively through the magnetic deflecting fields of the first and second deflectors that are positioned between the selector and the recording surface. It should be understood, of course, that the selection process can be reversed, so that droplets which are to be guttered are magnetized while the "selected" droplets are not magnetized by the selector and are aimed away from the gutter.
In order that the catcher may function effectively to intercept the unselected droplets, it has been customary heretofore in designing VMIJ printers to place the catcher between the first and second deflectors where it catches the unselected droplets before they are subjected to the second deflecting field. Experience gained in working with prior designs of such deflectors has dictated the necessity of placing the catcher in this position because if the unselected droplets were permitted to pass through both deflecting fields before being caught, their respective courses may be so widely divergent by the time they pass through the second deflecting field that it would be impossible to locate the catcher properly for intercepting all of the unselected droplets.
Placing the Catcher between the first and second deflectors has several disadvantages, however. First, it causes the spacing between the two deflecting fields to be greater than it should be for optimum control, so that by the time the stream of selected droplets reaches the second deflector, it is apt to have an undesirably large "spread" along the axis of the first deflecting field, thereby decreasing the likelihood that the second deflecting field can effectively control all of the droplets that have passed through the first deflector.
A second disadvantage of catching the unselected droplets before they can enter the second deflector is that it prevents the employment of an "alternate selection" principle whereby the droplets to be used in printing are effectively separated from each other by unselected droplets. For best results it has been found that the droplets which are to be used for printing should not follow one another too closely. If the selected droplets are traveling too closely together, the leading droplet often is overtaken by and merged with the droplet behind it before it reaches the recording surface, with adverse effect upon the printing quality. It has been observed also that undesirable magnetic interactions will take place between selected droplets and cause erratic movements of these droplets if they are not adequately spaced from each other at every point in their travel. There is a better opportunity to produce printing of high graphic quality if the selected and unselected droplets are alternately arranged as they leave the selector and are caused to travel concurrently in slightly diverging substreams. This produces the desired spacing between selected droplets along their course of travel, and with the two substreams traveling in angular proximity to each other, each contributes a certain amount of aerodynamic stability to the other. This advantage is lost, however, if the substream of unselected droplets is abruptly intercepted by the catcher as it leaves the first deflector so that the substream of selected droplets passes by itself into the second deflector. There appears to be some need for the two substreams to proceed together on their divergent courses through both deflecting fields in order to minimize aerodynamic disturbances and obtain a stabilized stream of selected droplets all the way to the recording surface. For some reason not fully known, stream stability is adversely affected if the catcher halts the progress of the unselected substream as it emerges from the first deflecting field, thereby forcing the substream of selected droplets to proceed alone to the second deflector.
Thus, designers of prior VMIJ printers have been confronted with a dilemma. In order to retrieve the unselected droplets effectively, they have considered it necessary to place the catcher between the first and second deflectors; otherwise, the unselected droplets would be so widely scattered by the second deflecting field in the direction of the second deflection axis that they could not be "guttered" effectively. By doing this, however, designers of such equipment have in several ways substantially reduced the probability of obtaining high quality printing using the VMIJ technique. The innate advantages of the vector magnetic ink jet technique over other ink jet printing techniques are such that it would be highly desirable to produce a VMIJ printing apparatus wherein the above described factors which tend to detract from high graphic quality are rendered negligible, thereby enabling users of such printers to realize their full potential for achieving high quality printing with minimum waste of ink.